Interest in the recovery of carbon dioxide (hereinafter “CO2”) from various CO2 containing gas mixtures has increased due to a variety of factors including the merchant CO2 market, enhanced oil recovery and greenhouse gas emissions reduction. The currently available systems for recovering high purity CO2 use a variety of generic and proprietary physical and chemical solvents such as conventional pressure swing adsorption units (hereinafter “PSA units”) and CO, recovery units downstream from the PSA unit such as amine liquid wash units, Selexol liquid wash units or methanol liquid wash units. Accordingly, the processes utilized for this recovery require a large investment due to equipment costs and also high regeneration energy requirements.
Carbon dioxide containing gas mixtures are produced as waste streams during the production of hydrogen gas from hydrocarbon streams using standard steam hydrocarbon reforming processes (hereinafter “SHR”). The most preferred of the SHR processes involves the production of hydrogen gas from hydrocarbon streams using steam methane reforming (hereinafter “SMR”) processes since methane has a higher proportion of hydrogen than other hydrocarbons. More specifically, with regard to general SMR processes, a hydrocarbon feed gas (natural gas) is fed into a SMR device where the methane in the feed gas reacts with steam at high temperatures (from about 700° C. to about 1100° C.) in the presence of a metal-based catalyst to produce a partially reformed gas that is a mixture of carbon monoxide and hydrogen. The hydrogen yield of this mixture is increased by passing the resulting mixture through a water gas shift reactor which promotes the conversion of carbon monoxide and water into more hydrogen. Accordingly, the result is a reformed gas stream that is rich in hydrogen but also contains to a lesser degree carbon dioxide, methane, and carbon monoxide. Such units typically operate at a temperature from about 200° C. to about 500° C. In some cases, the stream from the SHR will be at a higher temperature so optionally the stream may first be cooled with a heat exchanger before being passed through the water gas shift. In one conventional process, the reformed gas stream (the hydrogen rich stream) produced is then passed through a H2 pressure swing adsorption unit (hereinafter “H2 PSA unit”) in order to allow for the removal of from about 80% to about 90% or more of the hydrogen present through the use of adsorbents. The removal of the hydrogen results in a waste stream (also commonly referred to as a “PSA tail gas stream”) that is purged from the H2 PSA unit. This PSA tail gas stream contains methane, carbon monoxide, carbon dioxide, water, and any unrecovered hydrogen. This differs from the SHR units, with the difference being that the waste stream or tail gas produced in the SHR units contains alkanes of varying size (CnH2n+2) and water. The desire has been to be able to utilize these waste streams more efficiently as in the past they have simply been recycled to be burned as make up fuel (added to the natural gas used in the SHR process or SMR process).
Recently, a CO2 cryogenic process unit (hereinafter “CPU”) process was proposed to capture the CO2 during steam methane reforming H2 pressure swing adsorption off gas (by Air Liquide) in WO 2006/054008. In this process, the waste gas from the CPU plant, which normally contains significant amounts of H2, can be recycled back to the SMR plant for additional H2 production credit. The process requires operation at high pressure and cold temperature though. Therefore, while it may be appropriate to use the CO2 CPU process in a very large scale CO2 recovery plant (>1000 TPD), when applying the CO2 CPU process in a small size CO2 recovery plant (typically 100 to 500 TPD merchant CO2 plants), the energy and maintenance costs are considered to be usually high.
In an alternative conventional process, there is a CO2 recovery unit downstream of the H2 PSA. This CO2 recovery unit can be a liquid wash unit such as an amine liquid wash unit, a Selexol liquid wash unit or a methanol liquid wash unit or a cryogenic unit. In this schematic, the H2 PSA tail gas is optionally compressed upstream of the CO2 recovery unit. The tail gas after CO2 recovery is then recycled to the PSA unit, recycled as the SMR feed or used as fuel in the SMR furnace. By recycling the tail gas, H2 recovery is increased.
Even with the above conventional methods, there exists a need to provide a process that allows for a more economical recovery of highly concentrated CO2 from a pressure swing adsorption process without effecting hydrogen recovery.